Levothyroxine Dosage Calculator

Levothyroxine is not a drug in the traditional sense — it is a synthetic copy of T4, the hormone your thyroid gland would produce if it could. Dosing is essentially an impersonation exercise: estimate how much hormone a healthy thyroid would make for someone of your size, then replace that amount in pill form.

The math starts with body weight because thyroid hormone requirements scale with metabolic mass. A larger body has more cells demanding energy, and T4 regulates metabolism throughout every organ. The standard approximation for a person with no thyroid is 1.6 mcg per kilogram of body weight per day — meaning a 70 kg person without a thyroid needs roughly 112 mcg daily. That number is then adjusted downward for partial thyroid function, age, and cardiac risk, or upward for conditions like TSH suppression after thyroid cancer.

What the calculator cannot do is account for absorption variability, drug interactions (calcium, iron, and certain antacids all reduce absorption), or your individual TSH setpoint. Two people with identical weight and condition can end up on doses 50 mcg apart after titration. This estimate gets you to the right ballpark — your TSH at 6-8 weeks tells you where in that ballpark you actually land.

Use this calculator when you are newly diagnosed with hypothyroidism and want to understand what a weight-appropriate starting dose looks like before your appointment. It is also useful for patients who have been underdosed for years and want to check whether their current dose is in the right ballpark for their size and condition type.

This calculator is also relevant for people who have recently had a thyroidectomy or radioactive iodine ablation and are transitioning to full replacement. The 1.6 mcg/kg target gives you a clear endpoint to work toward, even if you start lower.

Do not use this calculator to adjust an existing dose — that requires TSH lab data, not arithmetic. Do not use it if you have kidney disease, severe heart failure, or are on medications known to interfere with levothyroxine metabolism (amiodarone, lithium, certain anticonvulsants). Do not use it for children or adolescents — pediatric dosing is weight-based but uses age-specific ranges that differ substantially from adult guidelines.

The most common mistake is using total body weight instead of lean body weight for people who are significantly overweight. Thyroid hormone requirements are driven by lean metabolic mass, not fat tissue. Using total weight in a 120 kg person with significant adiposity can generate a dose estimate 20-30 mcg higher than appropriate. If you carry significant extra weight, ask your doctor about lean body weight dosing.

The second mistake is assuming the starting dose is the final dose. It almost never is. Levothyroxine has a half-life of about 7 days, meaning it takes 4-6 weeks to reach a new steady state after any dose change. Patients who feel no improvement at week 2 and double their dose are dangerously ahead of the pharmacology. The standard titration interval is 6-8 weeks between changes.

The third mistake is timing the dose incorrectly, which is specific to levothyroxine in a way that matters to this calculator. Even if the dose is right, taking it with coffee, calcium, or within 4 hours of an iron supplement can reduce absorption by 30-50%. A patient on 100 mcg who absorbs only 60% effectively is on 60 mcg. If lab results are inconsistent, absorption timing is the first thing to examine — not the dose itself.

The core formula is: estimated dose (mcg) = dose factor (mcg/kg) x body weight (kg).

Dose factors by condition: full replacement uses 1.6 mcg/kg; standard hypothyroidism starts at 1.0 mcg/kg; subclinical hypothyroidism uses 0.5 mcg/kg; TSH suppression (post-cancer) uses 2.0 mcg/kg.

Age adjustments are applied as multipliers after the base calculation. Patients aged 50-65 have their dose reduced by 25%. Patients over 65 have their dose capped at 50 mcg regardless of weight. Cardiac disease caps the starting dose at 25 mcg regardless of all other factors. Pregnancy applies a 30% increase to account for elevated binding protein and fetal demand. Where multiple adjustments apply — for example, age 55 with cardiac disease — the more conservative cap takes precedence. The calculator also identifies the nearest commercially available tablet strength from the standard US formulary: 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, and 300 mcg.

The weight-based formula assumes linear scaling of thyroid hormone need with body mass, but this breaks down at extremes. In morbid obesity, the peripheral conversion of T4 to active T3 is impaired, and adipose tissue has lower metabolic activity per unit mass than muscle. Dosing to total weight overestimates the target. At the other extreme, in patients with very low body weight from muscle wasting, the formula can underestimate. The real target is metabolically active lean mass, which no calculator can infer from weight alone. Clinicians who see thyroid patients at weight extremes often use ideal body weight or adjusted body weight formulas as the denominator — a nuance that does not appear in standard guidelines but matters in practice.